1. Field of the Invention
The invention relates to a process and apparatus for optically reproducing and recording X-ray images.
2. Description of the Prior Art
The recording of X-ray images is widely carried out on photographic X-ray films and plates. In recent times images have been recorded by the xeroradiography process in which a photoconductor layer, which is charged before exposure and is preferably made of selenium, is partially discharged by X-rays; the residual charge image being made visible by a toner. The sensitivity of the selenium layers to X-rays is, however, only relatively slight.
In the process disclosed by German Offenlegungsschrift No. 2,436,894, X-ray images at a short access time are recorded on thermoplastic or photoconducting thermoplastic layers in an ionization chamber. For this purpose the thermoplastic recording material is placed on a transparent electrode in the ionization chamber, which is partially transparent to radiation. The chamber is subjected to X-rays which have passed through the object of which an image is to be made and which is located in front of the ionization chamber. A high voltage is applied to the electrodes of the ionization chamber which is filled with xenon under excess pressure. The charges formed in the ionization chamber, which are proportional to the X-radiation, are deposited on the thermoplastic recording layer. When the charge image, which is formed by the precipitation of the charges on the recording layer, is heated, it deforms the softened thermoplastic recording layer to give a relief image, as a result of the electrostatic forces emanating from the charges. The relief image is projected with schlieren optics as a continuous (half) tone image. For the reproduction of the continuous (half) tone image the thermoplastic layer can be provided with a screen by a periodic fine pattern, for example a grid pattern.
In the reproduction of X-ray images, which are recorded on silver film materials or are recorded as electrostatic images on appropriate layers developed with toner, grey gradations are obtained which must be interpreted with regard to their information content based on the experience on the radiographers evaluating them.
It is surprising that the literature describing the state of the art does not contain any mention of X-ray color images, in spite of the knowledge that relief images must be screened for continuous tone reproduction. It would appear that the steps in the process of reproducing X-ray images in color have not been discovered because of the fact that experts have always attempted only to reproduce the usual grey shade gradations by the diffraction effects which occur with the screen, by modulating up to the maximum value the intensities of the light deflected on the periodic structures into the first-order diffraction. Depending on the particular wavelength of the radiated light, this maximum value is reached, on wave-like periodic structures, with differences in the optical path lengths between 0.12 .mu.m to 0.2 .mu.m. With relief structures in organic thermoplastic polymers, this corresponds to deformation depths between 0.25 .mu.m and 0.4 .mu.m.
A further obstacle to the discovery of a process for the reproduction of colored X-ray images, may also have been that in the state of the art the work in general is carried out with monochromatic laser light and not with polychromatic light.
Attempts have been made (Bild der Wissenschaft, Sept. 1974, page 64) to convert the gray values into color steps for improved discernibility. For this purpose continuous (half) tone images are recopied on equidensity films into color images. However, as a result of the wet development which is necessary, the recopying process is time-consuming in addition to the time involved in the recording process itself.
Making color image recordings on positive photolacquer layers with the aid of rectangular grids is also known. Such processes have proven unsatisfactory when using X-rays, because photolacquer layers are for the most part insensitive to X-rays, so that very long exposure times would be necessary for making color image recordings. There is thus an obvious need in the present state of the art for color-image recordings.